CF is an autosomal recessive disorder characterized by chronic pulmonary bacterial colonization and progressive lung disease. By teh second decade of life, pseudomonas aeruginosa is the primary respiratory pathogen. This study is an open-label trial that uses aerosol administration of tobramycin solution to delivery high concentratinos of the drug directly to the site of infection. Patients will receive 300 mg/5mL tobramycin solution for inhalation twice daily for 28 days followed by 28 days of treatment. This treatment cycle will be repeated twice for a total of 3 cycles over a 6-month period.